In order to move an elevator car in an elevator shaft between different positions the car is suspended at a flexible supporting and/or drive means. In most recent times belts, apart from conventional steel cables, have also established themselves as supporting and/or drive means, which belts, for example, couple the elevator car with the counterweight and/or transmit a traction force for raising and lowering the car.
Knowledge of the position of the elevator car, thus its position in the elevator shaft, is required for control of the car. The speed or acceleration of the elevator car can also be determined from the position by differentiation according to time and can be similarly used in the control (for example the starting off or braking process or in the monitoring of a maximum speed/maximum acceleration), but also for, for example, determination of the actual car total weight as a quotient of force, which is exerted on the car by a drive means, and the resulting acceleration.
In order to determine the position of the elevator car, EP 1 278 693 B1 proposes a rotation transmitter which is arranged at the elevator car and which co-operates in mechanically positive manner with a separate cogged belt stretched in the shaft. This proposal requires, in disadvantageous manner, an additional cogged belt.
WO 2004/106208 A1 therefore proposes coding the support belt itself and detecting the position thereof by means of a detector arranged in the elevator shaft. The codings shall, according to the specification, preferably be realized by a magnetic material embedded in the belt, by changes (particularly enlargements) of wires arranged in the belt or by an additional cable in the belt and shall be contactlessly detected by an appropriate detector. WO 2004/106209 A1 expressly advises against grooves in the belt due to noise problems.
In the detection of the coding, as is proposed in WO 2004/106209 A1, the belt not only moves in correspondence with the movement of a elevator car, but can additionally move relative to the detector due to longitudinal, transversal and/or torsional oscillations induced by, for example, system inertias, movements of the car occupants or stick/slip effects in the guidance of the elevator car. Such additional movements of the belt are falsely detected by the detector as positional changes of the elevator car and falsify the positional determination. These errors amplify when the speeds or even accelerations are determined from the positions.
A further disadvantage of the system known from WO 2004/106209 A1 consists in that the proposed detectors, particularly optical or magnetic systems, need electrical energy and thus are no longer functionally capable in the event of damage, for example a fire, so that it is no longer possible to safely move the elevator car, with its help, to a predetermined position (for example an emergency disembarking position at the next storey or the ground floor), for example through the elevator being manually driven.
Finally, the systems proposed in WO 2004/106209 A1 are not optimal for the environmental conditions prevailing in an elevator shaft, particularly contamination or wear of the belt, since on the one hand the magnetic or optical coding can be diminished and on the other hand the sensitive detectors necessary for detection thereof can be damaged.
Proceeding from WO 2004/106209 A1 it is therefore an object of the present invention to provide a system and a method for detection of the position of an elevator car which is not impaired or is impaired only slightly by oscillations of the belt.